Laser sheet generator

ABSTRACT

A laser line generator having a laser diode, or other laser source, emits a laser which is passed through an aspherical lens to create a two-dimensional fan-shaped light sheet. This light sheet is reflected off a flat mirror to expand the width of the light sheet without increasing the size required by the optical path. The light sheet is then reflected by parabolic mirror to create a straight, planar light sheet.

RELATED APPLICATIONS

This application is a Continuation in Part of U.S. patent application Ser. No. 10/061,570, filed on Feb. 1, 2002, entitled “High Speed Laser Micrometer” which claims priority to Canadian Application No. 2,334,375, filed on Feb. 2, 2001, entitled “Laser Micrometer”, both of which are hereby incorporated by reference.

FIELD

The invention relates to laser measurement systems.

BACKGROUND

A laser micrometer provides dimensional information about objects placed in the path of a sheet of laser light which is detected by, or scanned across, a detector. The width of the “shadow” created on the detector provides a dimension of the object,

Existing “laser micrometers” are typically designed for small objects, with a maximum dimension of 6 inches. However, laser micrometers offer accuracy better than 1/1000^(th) of an inch, at a scan rate of up to a thousand samples per second. A

In a laser micrometer, a laser light sheet is usually formed using either static refractive lens elements to form a laser sheet or a rotating mirror to scan the beam. The limitation on the maximum size of the micrometer is the limitation on the size of these light sheet forming elements.

Alternative systems, known as “light curtains” are usually used in applications such as safety monitoring for preventing access to hazardous or secured areas. However, a laser curtain can be adapted to provide measurements of approximately ⅛^(th) of an inch accuracy and resolution. The scan rate from a light curtain is typically 100-200 samples per second.

In a light curtain system, the light sheet is typically a series of independent light beams emitted from a linear array of light emitting diodes (LEDs) spaced at the desired measurement resolution. An array of matching photodiode detectors completes the system. The light curtain design is limited in resolution by the physical spacing between the LEDs. The maximum scan rate is limited by the need to strobe the LEDs in segments to avoid crosstalk arising from adjacent photodiodes “seeing” the wrong LED. The maximum scan rate is also reduced as the size of the light curtain increases due to the large amount of data produced and the limitations of the typical interface and data encoding scheme.

Therefore, there is a need for a large format, high speed laser micrometer that is capable of scanning large objects with a high scan rate and high degree of accuracy.

It is an object of this invention to provide a large format, high speed laser micrometer to scan large objects with a high scan rate and a high degree of accuracy. It is an additional object of this invention to provide a parabolic mirror assembly for forming a collimated laser light sheet.

SUMMARY OF THE INVENTION

A laser line generator having a laser diode, or other laser source, emits a laser which is passed through an aspherical lens to create a two-dimensional fan-shaped light sheet. This light sheet is reflected off a flat mirror to expand the width of the light sheet without increasing the size required by the optical path. The light sheet is then reflected by parabolic mirror to create a straight, planar light sheet.

One application of the laser line generator is in a large profile, high speed laser micrometer. The micrometer is formed from a light source unit comprised of a plurality of emitter modules that combine to emit a laser sheet and a detector array comprised of a plurality of detector modules. Each of the emitter modules is aligned with a corresponding detector module such that an object passing between the light source unit and the detector array can be measured. Preferably, each of the emitter modules is comprised of two or more laser line generators arranged in an overlapping stair-step fashion to prevent gaps in the laser sheet emitted by the emitter module. Each of the detector modules is comprised of two or more linear CIS detectors, equal to the number of laser line generators, arranged in an overlapping stair-step fashion corresponding to said laser line generators.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself both as to organization and method of operation, as well as additional objects and advantages thereof, will become readily apparent from the following detailed description when read in connection with the accompanying drawings:

FIG. 1 is a plan view of the laser sheet mirror generator;

FIG. 2 is a plan view of the laser sheet generator;

FIG. 3 is a side view of the laser sheet generator;

FIG. 4 is schematic of a test setup for the laser sheet generator;

FIG. 5 is a front view of a large format laser micrometer; and

FIG. 6 is a side view of the laser line generator on an optical table.

DETAILED DESCRIPTION

The laser optics of the laser line generator 42 are shown in detail in FIGS. 1 and 2. Laser light 70 is passed through an aspherical lens 72 (e.g. Kodak Part # LG-11) to collect the light from the laser diode 68 and create a two-dimensional fan-shaped light sheet 80. This light sheet is reflected off a flat mirror 74 to expand the width of the light sheet without increasing the size required by the optical path. The light sheet is then reflected by parabolic mirror 76 to create a straight, planar light sheet. The term ‘parabolic mirror’ refers to a mirror curved in one plane to a parabola form—not to be confused with a paraboloidal mirror which is curved in two planes (although the term parabolic is often used to describe a paraboloidal mirror).

The laser diode 68 and lens 72 are supported on a laser plate 84. The angle of the laser plate 84 can be precisely adjusted so as to target the laser and to control the angle of incidence at mirrors 74 and 76. The laser plate 84, diode 68, lens 72, flat mirror 74 and parabolic mirror 76 are all supported on channel 86. Channel 86 can be fixed or mounted in a number of different ways depending on the application for which the laser line generator 42 is to be used.

Referring to FIG. 4, in one embodiment the laser line generator 42 has a thin first surface mirror 76 which is formed against a 20″ radius cylinder or parabola. The mirror is secured to a 1/16″ FR-4 backing plate using Durabond E-20NS epoxy. Two methods of making this mirror are presently feasible: vacuum forming against the appropriate form, and clamping to the form by securing end points of the mirror only. The laser diode 68 is set at a distance from the mirror 76 for best collimation, near the focus of 10″. The collimated laser light illuminates the target 100, and the resulting shadow of the target is viewed against the screen 110. The target 100 consists of a packaging strip of surface mount resistors which gives a line of 1 mm holes spaced at 4 mm apart. The screen 110 consists of a vertical foamcore sheet with an attached plot of vertical lines spaced 4 mm apart against which the shadow of the target can be evaluated. The deviation from true collimation is recorded, and then the target 100 is shifted laterally by 2 mm to increase the resolution of the measured collimation performance to an effective sampling interval of 2 mm.

Referring to FIG. 6, the alignment of one embodiment of the laser line generator 42 is described, using a procedure to obtain a collimated 8.5″ wide sheet of laser light. The laser line generator 42 is assumed to be complete, with the exception of the parabolic mirror 76 and final alignment, i.e. all spacers are yet to be inserted under the laser plate, the laser has yet to be secured to the laser plate, etc. The alignment consists of aligning the laser plate angle and position, and adjusting the parabolic mirror assembly to obtain a collimated sheet of light. The laser plate 84 must be aligned close to the correct, final angle. The final position and angle of the laser plate 84 will vary slightly due to differences in the parabolic mirror 76 (e.g. backing plate/mirror angle) and in the case of slight angular offsets in the flat mirror 74 on the emitter channel 86. For efficiency, a batch of several laser line generators 42 should be aligned at a time:

-   1) Mount the channel 86 to the steel angle fixture 120 on the     optical table 130; -   2) Adjust the laser plate 84 position for a nominal centred     position, with no skew angle to the laser diode 68 and lens 72. -   3) Power on the laser diode 68; -   4) Place the alignment target 140 adjacent to the edge of the     channel 86 and verify the laser sheet 82 is exiting close to the     first horizontal line on the target 140. -   5) Move the alignment target to 400 mm range from the channel 86. -   6) Adjust the angle of the laser plate 84 until the laser sheet 82     rises 19.3 mm ±0.5 mm over 400 m from the edge of the channel 86.

The laser line generator 42 is to give a well collimated (but slightly diverging) laser sheet 82. There should also be no skew in the plane of the laser sheet 82.

The forward/backward position of the laser plate 84 adjusts the collimation of the laser sheet 82, while the lateral (side/side) or angular position (skew) of the parabolic mirror 76 adjusts the centre of the laser sheet 82 and the skew angle of the laser sheet 82.

In one embodiment of the invention the parabolic mirror 76 consists of a thin glass mirror epoxied to a steel backing plate. The mirror 76 is formed to a parabolic profile by a combination of the steel backing plate which is machined to the parabolic form and an aluminum parabolic form against which the mirror 76 is pressed during the epoxy process.

The large format laser micrometer 10 shown in FIG. 5 is defined by a light source unit 20 and a detector array 30. The light source unit 20 is connected to a power supply (not shown) for activating and deactivating the light source unit 20. The detector array 30 is connected to a central processing unit (CPU) 34 (not shown) for receiving and interpreting data received from the detector array 30. The CPU 34 may be a dedicated hardware unit provided with a summary display of the micrometer output, a personal computer (PC) or a proprietary system. More generally, any system that interprets and presents the results, preferably while providing control options to the user, will suffice.

The light source unit 20 is comprised of a number of emitter modules 40. Each emitter module 40 has at least one laser line generator 42 (see FIGS. 1-3). In one embodiment the laser line generators 42 are arranged in a stair-step configuration with a slight overlap to eliminate any gaps between the laser line generators. Each laser line generator 42 forms a sheet of light equal in width to the laser line generator 42. The overall result is a laser sheet with an effective width equal to that of the emitter module 40.

In normal operation, each emitter module 40 will be either on (emitting light) or off. However, a pulsed mode of operation should also be provided to allow for alignment and adjustment of the detector array 30 at a lower (less than saturated) signal level.

The detector array 30 is comprised of a number of detector modules 50. Each detector module 50 is comprised of a number of linear CIS detectors arranged in a stair-step configuration to match the laser line generators 42 in the corresponding emitter module 40. An optical filter covers the CIS detectors to prevent signal interference from ambient or stray light sources.

The detector array 30 also includes one or more data processing units 32. As shown in FIG. 5, one data processing unit 32 is connected to three detector modules 50. The data processing units 32 are used to receive, interpret, and transmit the signals from the detector modules 50 to the CPU.

An object 16 passing between the light source unit 20 and the detector array 30 causes an interruption in the path of the laser light incident on the detectors. The resulting transition in the detector is recorded by the data processing unit 32 and passed to the CPU (not shown). The CPU then interprets the transition data and reports it to the user, either in a raw form, or as a calculated measurement of size, whichever is required.

While each detector module 50 may include its own data processing unit 32, it is preferable to have more than one detector module 50 coupled to a data processing unit 32, to reduce cost and system bandwidth requirements. In one configuration, there are two “slave” detector modules coupled to a “master” detector module, one to either side. The “master” detector module houses the data processing unit 32, which receives detector signals from the “master” unit and the two adjoining “slaves”.

While more “slaves” can be connected to one “master”, there will be a threshold based on the available data bandwidth for transmitting signals. If the number of “slaves” is too large, there will be signal loss at the data processing unit and gaps or errors will result. In a similar vein, while a separate data processing unit 32 could be used for all detector modules 50, the data bandwidth requirements for the CPU 34 make this configuration unsuitable for a detector array 30 with a large number of detector modules 50. The described array using one “master” with two “slaves” represents a balanced approach that should work with the majority of detector array configurations.

The data processing unit 32 is the interface between the detector array 20 and the CPU. The data processing unit 32 receives timing signals and commands from the CPU and transmits transition data and gray-scale “video” (if required) back.

With a standard detector at 200 dots per inch (dpi) resolution, the detector resolution is 0.005″ per pixel. The practical limit on the resolution is determined by the collimation quality of the laser light sheet and ambient or spurious lighting effects on edge definition. It may be preferable for the data processing unit to ignore every other pixel to reduce the number of spurious transitions and the data bandwidth requirements. The result is an effective resolution of 0.01″.

The data processing unit 32 requires some logic to account for conditions that produce a large number of transitions in a single scan line. For example, if a sharp edge of the object being measure is coincident with the longitudinal axis of one of the detectors it would result in a gray edge, a series of pixels rapidly exchanging between ON and OFF states, producing numerous transitions reported from the data processor. This could result in an overload of the data buffers and a consequential loss of data from the scan line and subsequent scan lines.

A second potential problem is excessive or false triggering resulting from interference from dust and other small particulates. Again, the repeated random transitions could overload the data buffers and result in a loss of subsequent data.

The solution is to provide for a user-defined value for transitions below which the transition should be ignored. For example, setting the value to one means that single pixel transitions are ignored i.e. a neighboring pixel must also undergo a transition at the same time for the transition to be recorded and the transition data transmitted.

The data buffer problem must also be considered in the context of available bandwidth both to and from the data processing unit 32. If each detector module 50 has a data processing unit 32, bandwidth to the data processing unit 32 is not a problem, however, bandwidth from the data processing unit 32 (to the CPU) becomes a larger factor. The combination of “master” and “slave” detector modules alleviates the situation, however, too many “slaves” and not enough “masters” creates the opposite scenario, in that bandwidth to the data processing unit 32 is now at a premium, and bandwidth from is not a concern.

The limitations on the system, therefore, lie in the bandwidth capabilities of the data processing unit 32 and CPU. One “master” and two “slaves” is presented herein as a example that provides efficient data handling capabilities. Obviously, systems with a higher bandwidth can use a larger “slave” to “master” ratio, which will permit a larger array, fewer “masters” to reduce cost, or both.

The laser micrometer 10 is intended for profiling, inspection, and process control applications where the outside profile and/or location and size of holes is required. Typical applications are:

Large punched sheet metal part inspection

Web guiding and detection of transmissive defects

Lumber gaging—board length and width, log diameter

The laser micrometer 10 provides significant advantages when compared to existing discrete photodiode based systems:

No crosstalk between sensors that limits measuring resolution and scan rate

True 0.005″ pixel spacing

2 KHz scan rate independent of system width

36 edges detected simultaneously per 72″ of width in standard configuration—more in custom versions.

In the preferred embodiment of the laser micrometer 10 the emitter modules 40 produce a visible light laser sheet made up of three overlapping 8.5″ wide collimated (<3mr) beams, which provide complete 24″ wide coverage per module.

In the preferred embodiment the detector modules 30 are made up of 200 dpi CMOS arrays, a master module with an optional slave unit on either side, and fibre optic control and data connection between the master module and the hub.

In the preferred embodiment of the micrometer 10 the hub is made up of a Motorola MPC8260 PowerPC based motherboard, a VxWorks Operating System, a fibre optic interface board which supports up to 4 Master detector modules, a fibre optic quadrature encoder and choice of two fibre optic object ‘detect’ inputs, a fast Ethernet connection (RJ-45 or optional fibre optic) to an NT host.

The laser line generators 42 may be useful, individually or in combination, in a number of applications beyond the above-described micrometer.

Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the scope of the invention. 

1. An apparatus for emitting a linear, planar sheet of light, comprising: a laser which emits a beam of light; an aspherical lens which converts said beam of light into a fan-shaped sheet of light; and a parabolic mirror which reflects said fan-shaped sheet of light into a linear, planar sheet of light.
 2. The apparatus according to claim 1, further comprising a flat mirror located between said aspherical lens and said parabolic mirror which reflects said fan-shaped sheet of light from said aspherical lens into said parabolic mirror. 